Transistor amplifier with high undistorted output



Oct. 5, 1954 R. F. SCHWARTZ 2,691,075

TRANSISTOR AMPLIFIER WITH HIGH UNDISTORTED OUTPUT Filed June 27. 1950INVENTOR @cf/95a wwaerz ATTORNEY Patented Oct. 5, 1954 TENT OFFICETRANSISTOR AMPLIFIER WITH HIGH UNDISTOR-TED OUTPUT Richard F. Schwartz,Gibbsboro, N. J., assigner to Radio Corporation of America, acorporation oi Delaware Application .lune 27, 1950, Serial No. 170,601

7 Claims. l

This invention relates generally to distortion ampliers, and moreparticularly relates to both single-ended and push-pull ampliner systemsof the semi-conductor type which have a higher undistorted output thanpreviously known semi-conductor ampliers.

Semi-conductor ampliiiers or transistor amplifiers are well known in theart. It is also known that they have a certain amount of signaldistortion which increases with the signal The distortion is partly dueto the nonlinear voltage-current characteristic of the sitter of theamplifier. This may be explained a small variation of the equivalentemitter that is, the resistance looking into the emitter electrode,which varies slightly as t signal goes through one electrical cycle. ithe absolute variation of the emitter tion of the output signal.

It is aise well known that a push-pull ampli-ner system has greaterpower output and less distortion than a single-ended amplifier. However,it is very difficult to manufacture transistors with matching electricalcharacteristics which rnay he used in a push-pull amplier system.iiccordingly, it would be desirable to provide some means forneutralizing the eilect of the diierent dynamic input characteristics oftwo transistor units in a push-pull amplifier system.

lt is accordingly an object of the present inn vention to provideimproved single-ended or push-pull semi-conductor amplifier systemshaving substantially less signal distortion than previously knownampliers of this type.

A further object of the invention is to provide simple circuit means ina single-ended or pushamplifier of the semi-conductor type whereby alig-her undistorted output signal may be obtlian with known ampliers cithis type.

Another object of the invention is to provide an improved push-pullsemi-conductor amplifier system which will permit the utilization of twoseinimconductor amplifier units of diierent electrical characteristics.

In accordance with the present invention a resistive impedance elementis eectively connected in the alternating-current input or outputcircuit of a semi-conductor amplifier system. The resistive impedanceelement has such a resistance as to reduce substantially signal distortn of the system. In other words, the signal distortion may he reducedwith the same power output or higher power output may be obtained low.il without increasing the signal distortion. In accordance with oneembodiment of the present invention a capacitor and a resistor areconnected in series across a portion of a direct current path whichsupplies bias 'voltages to either the emitter or to the collectorelectrode of the ampliiier system. The capacitor presents a 10Wimpedance to the input signal while the resistor has such a resistanceas to reduce signal distortion. Alternatively, a resistor may beprovided in the primary winding of an input transformer which hasa'secondary winding connected between base and emitter of the ampliersystem.

In a push-pull amplier system, in accordance with the present invention,a resistive mpedance element is connected effectively in one of thecommon alternating-current input or output circuits of the system. Sucha resistive impedance element will also balance the unequal electricalcharacteristics of the two transistors of the amplifier system. It is,of course, also feasible to provide a separate linearizing resistor inthe alternating-current input or output circuits of each amplier of thepush-pull ampliier system.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawing, in which:

Figure 1 is a circuit diagram of a single-ended semi-conductor amplifiercircuit embodying the present invention;

Figure 2 is a circuit diagram of a modified single-ended amplifiersystem in accordance with the invention;

Figure 3 is a circuit diagram of a push-pull amplier circuit whereinmeans are provided in v accordance with the invention for balancing thecircuit for the dissimilar electrical characteristics of the twosemi-conductor amplifiers of the circuit;

Figure 4 is an equivalent circuit diagram of the push-pull amplier ofFigure 3' which will be referred to in explaining the noise ordistortion balancing action of the circuit of Figure 3;

Figure 5 is a circuit diagram of a push-pull amplifier systemrepresenting a further embodiment of the invention; and

Figure 6 is a circuit diagram of a semi-conductor push-pull amplifiersystem. also in acf- 3 cordance with the invention, showing the use ofseparate means for reducing the distortion in each semi-conductoramplifier' of the system.

Referring now to the drawing, in which like components have beendesignated by the same reference numerals throughout the figures andparticularly to Figure 1, there is illustrated a single-ended amplifiersystem in accordance with the invention comprising a semi-conductingbody i0. Body I6 may consist of a semi-conducting crystal such, forexample as germanium or silicon containing a small number or impuritycenters or lattice imperfections. The surface of body I3 may be polishedand etched as is conventional in the art. Body Iii may consist of N typegermanium although it is feasible to use P type germanium. For thefollowing discussion it will be assumed that body I6 is of the N type.

Emitter electrode II, base electrode I2 and collector electrode I3 arein contact with body Hl. Base electrode I2 is in low-resistance contactwith body IB and may, for example, be a largearea electrode. Thus, thebase electrode may be represented by a suitable piece of metal such asbrass which has been soldered to body Il). Emiter electrode Il andcollector electrode I3 are in rectifying Contact with body I0. They maybe small-area electrodes such as point or line contact electrodes whichmay consist, for example, of tungsten or Phosphor bronze wires. Ifelectrodes II and I3 are point electrodes, the wire ends may be pointed.

For the purpose of applying operating potentials to the semi-conductordevice a bias voltage in the forward direction is applied betweenemitter II and base I2 while a bias voltage in the reverse direction isapplied between collector i3 and base I2. Accordingly, emitter electrodeli should be positive with respect to base I2 while collector electrodeI3 should be negative with respect toV base I2. If body I consists of aP type crystal, the potentials must be reversed. A suitable source ofvoltage such as battery I4 is provided for applying a bias voltage toemitter electrode II. To this end the negative terminal of battery I4 isgrounded while its positive terminal is connected to emitter electrodeII through resistor I5. Battery I6 has its positive terminal groundedwhile its negative terminal is connected through resistor I1 tocollector electrode I3. Base electrode I2 is grounded.

A signal to be amplified is developed by signal source 20 having oneterminal grounded while its other terminal is coupled to emitterelectrode II through coupling capacitbr 2|. The amplified output signalis developed across load resistor I1 and may be otbained from outputterminals 22, one of which is grounded while the other one is coupledthrough coupling capacitor 23 to collector electrode I3.

The amplifier of Figure l as described herein is conventional. The inputsignal is impressed effectively between emitter electrode II and baseelectrode I2 while the amplified output signal is developed effectivelybetween collector electrode I3 and base electrode I2. It is alsoconventional practice to bypass batteries I4 and I6 by capacitors 24 and25, respectively, which have a low impedance for signal-frequencycurrents.

In accordance with the present invention a linearizing resistor 26 isprovided in series with capacitor 24 both being connected across batteryI4. Another linearizing resistor 21 may be provided in series withcapacitor 25 and connected across battery I6. Linearizing resistors 26and 21 are accordingly in the altermating-current input circuit andoutput circuit respectively. Batteries I4 and I6 should have anappreciable impedance for signal frequency currents so that theimpedance of battery I4, for example, is larger for alternating currentsthan that of the shunt path 24, 26. To this end adjustable resistors 28and 29 have been shown in series with batteries I4 and I6, respectively.The resistance of resistors 28 and 29 may be ten times that of resistors26 and 21. It is to be understood that either resistor 2S or 21 may beomitted. Preferably, however, the amplifier system of the invention isprovided with linearizing resistor 26 in the emitter circuit whileresistor 21 in the collector circuit is optional. The magnitude of theresistance of resistors 26 and 21 determines the distortion of theoutput signal. Thus, the distortion may be substantially reduced by theprovision of resistors 26 and 21 with the same output power.Alternatively, a higher output power may be obtained without increasingthe signal distortion.

As explained previously, it is believed that the signal distortion isdue to small variations of the resistance which appears looking intoemitter electrode II. This distortion is substantially reduced by theprovision of resistor 26, for example, which is provided only in thealternating-current input circuit of the amplifier. The resistance ofresistor 26 or of resistor 21 does not have an optimum value. The largerthe resistance of resistor 26 or 21, the smaller the distortion but alsothe higher will be the power required to drive the amplifier. This willbe understood more clearly from the following table showing typicalresults for an amplifier system in accordance with the invention havinga transformer input circuit (see Figure 2) and where resistor 21 hasbeen omitted.

Percent Distortion Rfsistauce of Rcsistvr 26 Output Voltage Output PowerInput Voltage 5H() ohms "l:

ser#

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It will accordingly be seen that the distortion is reduced toapproximately one-quarter by the provision of resistor 26 having aresistance of 500 ohms. At the same time, however, the input voltagemust be increased approximately iive times to obtain the same poweroutput. Thus, the higher the resistance of resistor 26, the less thedistortion will be but a point is eventually reached where thedistortion levels off. The distortion level is determined by thedistortion of the input circuit and of signal source 20. Finally, theoutput power merely decreases as the resistance of resistor 26 isincreased.

It has been found that the collector current is substantially linearwith the emitter current but not with the emitter voltage. Thus, whenthe alternating emitter current which is the current supplied by signalsource 20 is linearized, the collector current is linearized thereby toreduce the signal distortion.

It is also feasible to reiiect the resistance provided in the primary ofa transformer into the transformer secondary thereby to provideeffectively a resistance in the alternating-current input circuit of theamplifier. Such an arrangement is illustrated in Figure 2. Inputtransformer includes primary winding 3l and secondary winding 32.Resistor 33 is connected in series with input terminals 20 and primarywinding 3l. Secondary winding 32 is connected between battery I4 andemitter II. Output transformer 34 includes primary winding 35 andsecondary winding 3&5. Primary winding 35 is connected between collectorI3 and battery I6. Secondary winding 3S is connected across outputterminals 22. Output load resistor 31 is connected in shunt withsecondary winding 36.

The circuit of Figure 2 operates substantially in the same manner as thecircuit of Figure 1. The reflected resistance of resistor 33 appears inseries with secondary winding 32. This resistance is, of course, not inthe direct current path including battery I4, secondary winding 32 andemitter II. The resistance of resistor 33 may be selected in the samemanner as that of resistor 26 or 21 of the circuit of Figure 1 and asimilar reduction of the distortion results.

A push-pull amplifier circuit in accordance with the invention isillustrated in Figure 3. The push-pull amplifier system includes twosemiconducting bodies I@ and I. A base electrode I2, I2', an emitterelectrode II, II and a collector electrode I3, I3 are in contact witheach body l0 and I4 respectively. The two base electrodes I2 and I2 aregrounded as shown. Battery I4 and resistor 28 in series are connectedbetween ground and the two emitter electrodes through resistors 4i) and4l respectively. Battery I5 and resistor 29 are serially connectedbetween ground and the midpoint of primary winding 42 having a secondaryor output Winding 43. Load resistor 31 is connected in shunt withsecondary winding 43, and the output signal may be derived from outputterminals 22. A push-pull input signal is impressed on input terminals44 which, as shown, are balanced with respect to ground. The push-pullinput signal is impressed on emitter electrodes II and II throughcapacitors 45 and 46.

Battery I4 and resistor 28 and battery I6 and resistor 29 are againbypassed for signal-frequency currents by capacitors 24 and 25,respectively. In accordance with the present invention resistors 41 and48 are connected in series with capacitors 24 and 25 respectively acrossbatteries I4 and I6.

Resistors 41 and 48 serve the same function as resistors 26 and 21 inFigure 1. Additionally, resistors 41 and 48 compensate for the inherentdifferences in the characteristics of the two ampliers I0 and I0. indetail by reference to Figure 4 which is an equivalentalternating-current circuit of the push-pull amplifier of Figure 3. Asshown in Figure 4, resistor 41 is in the common alternating currentemitter circuit of the two amplifiers I0 and I4. Generator 50 representsthe noise or distortion voltage produced by the circuit of emitterelectrode II. This voltage will cause a current to flow in the directionindicated by arrow 5I thereby producing a voltage drop across resistor41.

Thus, if the terminal of generator 53 connected to emitter Il ispositive, a negative voltage will be developed at the junction point or"resistors 40, 4I with resistor 41. This negative voltage will beimpressed on emitter electrode II. Thus, a voltage in phase oppositionis impressed on emitter electrode II which counteracts the originaldistortion voltage developed by generator 50. Therefore, the distortionpro- This will be explained more I duced by each amplifier l0 or I0'injects an opposing distortion voltage into the other amplifier. Adenite optimum value for the resistance of resistor 41 or 48 exists.This is due to the fact that the resistance needed to cancel oneharmonic term completely is not the same as the value needed to cancelanother harmonic term completely. It has been found experimentally thatthe resistance of resistor 41 should be of the order of to 200 ohms. Thefollowing table gives some information on the reduction of distortiondue to the provision of resistor 41, resistor 43 being omitted. Thetests were taken with a transformer input circuit as shown in Figure 5.

It will therefore be seen that the distortion has been reduced more than12 times while the driving voltage had to be increased by a factor ofapproximately 2. Thus, the improvement in distortion in the push-pullcircuit does not require as much increase in the driving power as doesthe single-ended driving circuit.

Figure 5 illustrates a modified push-pull ampliiier in accordance withthe invention. The input circuit includes transformer 55 having aprimary winding 56 and a secondary winding 31. Linearizing resistor 58is connected in series with input terminals 44 and primary winding 55.The midpoint of secondary winding 51 is connected to battery I4 whileits terminals are connected to emitter electrodes II and II. The twobase electrodes I2 and I2 are grounded as shown. The output circuit issimilar to that shown in Figure 3.

In the push-pull amplifier system of Figure 5 the resistance of resistor5S is reected into the alternating-current input circuit. In otherwords, the reflected resistance appears in series with the secondarywinding 51 in the alternating-current circuits of both emitterelectrodes II and Il. The effect of resistor 58 is similar to that ofresistor 33 in the single-ended amplifier circuit of Figure 2.

The push-pull amplifier circuit of Figure 6 permits an individualadjustment of the directcurrent bias voltages applied to both amplifiersI3 and I0 and also an individual adjustment of the resistance in thealternating current input and output circuits of each ampliiier. Thus,battery I4 is connected through resistor 6i) and winding 6I to emitterelectrode EI. Battery I4 is also connected through resistor 62 andwinding 63 to emitter` electrode I I. The input signal is impressedthrough terminals 44 on primary winding 64 which is inductively coupledto windings 5I and 63 which represent the secondary windings of theinput transformer. Battery i4 and resistor t4 are shunted by resistor 65and capacitor 66 connected in series. Capacitor 5S bypassessignal-frequency currents while resistor 65 functions as the linearizingresistor. Similarly, battery I4 and resistor 62 are shunted by resistor61 and capacitor B8 connected in series. Capacitor 65 bypassessignal-frequency currents and resistor @1 again functions as thelinearizing resistor for amplifier lll. Resistors t3, t2, 65 and 61 maybe adjustable as shown to adjust the emitter bias voltages and theresistance of the linearizing resistors.

Collector battery I6 is connected through resistor l and winding H tocollector electrode i3. Similarly, battery It is .connected throughresistor 12 and winding 13 to collector electrode i3'. Capacitor M andresistor 15 connected in series are shunted across battery i6 andresistor Til. Furthermore, capacitor .16 and resistor 'l'l are shuntedacross battery I6 and resistor' l2. Resistors l0 and l2 may beadjustable to adjust the collector bias voltages. Linearizing resistorsI5 and 'l1 may also be adjustable as shown. Windings 1| and T3 areinductively coupled to output winding i3 across which load resistor 31is shunted. The output signal may be obtained from output terminals 22.

It is to be understood that linearizing resistors 65, 6l, l5 and 17 areof the order of 200 to 500 ohms while resistors 60, 62 and '10, 'I2 areofthe order of 2,000 ohms or more. In view of the previous explanationsthe operation of the pushpull amplifier system of Figure 6 will beevident. It is to be understood that linearizing resistors 'l5 and Ilmay be omitted if desired.

The push-pull amplier system of Figure 6 may also be modied to provide asingle linearizing resistor for both emitter circuits and another singlelinearizing resistor for both collector circuits, To this end capacitor66 should be connected to the junction point of battery ld withresistors 60, 62. Battery I4 should have a high alternating currentimpedance which may be obtained, for example, by connecting a choke coilin series with the battery. Capacitor 56 and resistor 65 will then shuntthe high impedance voltage source Ill. Similarly, capacitor 'Hl andresistor 'l5 may be connected in shunt with battery I6 which should havea high alternating current impedance which may be obtained by connectinga choke coil in series with battery I6.

There have thus been disclosed single-ended and push-pull semi-conductoramplier systems having Va high undistorted output. The signal distortionis considerably reduced with the same output power or the output powermay be increased without increasing the distortion as compared topreviously known semi-conductor amplifiers. Push-pull amplier circuitshave been disclosed which permit the utilization of two semi-conductoramplier units with dissimilar electrical characteristics. the reductionof the signal distortion is appreciable while the required input poweris only slightly increased.

What is claimed is:

1. An amplier system comprising a semiconducting body, a base electrode,an emitter electrode and a collector electrode in contact with saidbody, a iirst source of voltage connected between said emitter and baseelectrodes in such a polarity as to impress a voltage on said electrodesin the forward direction, a signal frequency by-pass capacitor and aresistor connected in series across said first source, a second sourceof voltage connected between said collector and base electrodes in sucha `polarity as to impress a voltage on said electrodes in the reversedirection, an input circuit coupled between said emitter and baseelectrodes, and an output circuit coupled between said collector andbase electrodes, said resistor having such a resistance as to reducesubstantially signal distortion of said amplifier system.

2. An amplifier system comprising -a semiconducting body, a baseelectrode, an emitter electrode and a collector :electrode in ycontactIn that case,

with :said body, a first source of voltage connected between saidemitter and base electrodes in such a polarity as to impress a voltageon said electrodes in the forward direction, a second source of voltageconnected between said collector and base electrodes in such a polarityas to impress a voltage on said electrodes in the reverse direction, avsignal frequency by-pass capacitor and a resistor connected in seriesacross said second source, an input circuit coupled between said emitterand base electrodes, and an output circuit coupled between saidcollector and base electrodes, said resistor having such a resistance asto reduce substantially signal distortion of said amplifier system.

3. .An amplifier system comprising a semiconducting body, a baseelectrode, an emitter electrode and a collector electrode in contactwith said body, a first source of voltage connected between said emitterand base electrodes in such a polarity as 'to impress a voltage on saidelectrodes in the forward direction, a rst signalfrequency by-passcapacitor and a first resistor connected in series across said rstsource, a second source of voltage connected between said collector andbase electrodes in such a polarity as to impress a voltage on saidelectrodes in the reverse direction, a second signal-frequency bypasscapacitor and a second resistor connected in series across said secondsource, an input circuit coupled between said emitter and baseelectrodes, and an output circuit coupled between said collector andbase electrodes, said resistors having such a resistance as to reducesubstantially signal distortion of said amplifier system.

4. A push-pull amplifier system comprising a pair of semi-conductingbodies, a base electrode, an emitter electrode and a collector electrodein contact with each body, a conductive connection between said baseelectrodes, a iirst direct current path between said base electrodes,and each of said emitter electrodes, said rst path including a rstportion common to both emitter electrodes, means connected in said rstportion for applying la bias voltage in the forward direction betweeneach of said emitter electrodes and said base electrodes, a seconddirect current path connected between said base electrodes and each ofsaid collector electrodes, saidsecond path including a second portioncommon to both collector electrodes, means connected in said secondportion for applying a bias voltage in the reverse direction `betweeneach of said collector electrodes and said base electrodes, asignalfrequency by-pass capacitor and a resistor connected'in seriesacross one of said portions, said resistors having such a resistance asto reduce substantially the signal distortion of said amplifier systemand to balance the unequal electrical characteristics of said twobodies, a signal input circuit coupled between said emitter electrodes,and a signal output circuit coupled between said collector electrodes.

5. A push-pull amplier system comprising a pair of semi-conductingbodies, a base electrode, an emitter electrode and a collector electrodein contact with each body, a conductive connection between said baseelectrodes, a direct current path connected between said base electrodesand each of said emitter electrodes, said path including a portioncommon to rboth emitter electrodes, means connected in said portion forapplying a bias voltage in the forward direction between each oisaidemitter electrodes and said base electrodes, means lforapplying a biasvoltage in the reverse direction between each of said collectorelectrodes and said base electrodes, a signal-frequency by-passcapacitor and a resistor connected in series across said portion, saidresistor having such a resistance as to reduce substantially the signaldistortion of said ampliiier system and to balance the unequalelectrical characteristics of said two bodies, a signal input circuitcoupled between said emitter electrodes, and a signal output circuitcoupled between said collector electrodes.

6. A push-pull ampliiier system comprising a pair of semi-conductingbodies, a base electrode, an emitter electrode and a collector electrodein contact with each body, a conductive connection between said baseelectrodes, means for applying a bias voltage in the forward directionbetween each of said emitter electrodes and said base electrodes, adirect current path connected between said base electrodes and each ofsaid collector electrodes, said path including a portion common to bothcollector electrodes, means connected in said portion for applying abias Voltage in the reverse direction between each of said collectorelectrodes and said base electrodes, a signal frequency by-passcapacitor and a resistor connected in series across said portion, havingsuch a resistance as to reduce substantially the signal distortion ofsaid amplifier system and to balance the unequal electricalcharacteristics of said two bodies, a signal input circuit coupledbetween said emitter electrodes, and a signal output circuit coupledbetween said co1- lector electrodes.

7. A push-pull amplifier system comprising a pair of semi-conductingbodies, a base electrode, an emitter electrode and a collector electrodein lil contact with each body, a conductive connection between said baseelectrodes, a rst direct current path connected between said baseelectrodes and each of said emitter electrodes, said first pathincluding a rst portion common to both emitter electrodes, meansconnected in said first portion for applying a bias voltage in theforward direction between each of said emitter electrodes and said baseelectrodes, a second direct current path connected between said baseelectrodes and each of said collector electrodes, said second pathincluding a second portion common to both collector electrodes, meansconnected in said second portion for applying a bias voltage in thereverse direction between each of said collector electrodes and saidbase electrodes, a first signal-frequency by-pass capacitor and a iirstresistor connected in series across said first portion, a secondsignalfrequency by-pass capacitor and a second resistor connected inseries across said second portion, said resistors having such aresistance as to reduce substantially the signal distortion of saidamplifier system and to balance the unequal electrical characteristicsof said two bodies, a signal input circuit coupled between said emitterelectrodes, and a signal output circuit coupled between said collectorelectrodes.

References Cited in the iile of this patent UNITED STATES PATENTS NumberName Date 2,273,432 Brewer Feb. 17, 1942 2,541,322 Barney Feb. 13, 19512,609,459 Bergson Sept. 2, 1952 2,647,957 Mallinckrodt Aug. 4, 19532,647,958 Barney Aug. 4, 1953

